Neuroscience For Kids

Visual Pathway

(Image courtesy
National Library of Medicine, Bethesda, Maryland.)

If you can understand the lower figure on this page, then you have
learned the major pathway that visual information takes on its way from
the eye to the primary visual cortex. The figure is really not too
difficult to understand....really.

Imagine that the colored bar (half red, half blue) is in front of your
eyes. The red part of the bar will project to the nasal part of your
left retina and the temporal (lateral) part of your right
retina. The blue part of the bar will project to the nasal part of
your right retina and the temportal (lateral) part of your left
retina.

Right and left visual
information cross to opposite sides of the brain. This crossover occurs
in the optic chiasm. After the optic chiasm, information about the right
visual field (blue) is on the left side of the brain, and information
about the left visual field (red) is on the right side. The pathways stay
this way all the way up to the visual cortex.

Follow the blue and red lines from the eyes to see the flow of
information. From the retina, the first synapse is in the lateral
geniculate nucleus of the thalamus. The next synapse is made in primary
visual cortex in the occiptal lobe.

The Visual Pathway: From Eye to Primary Visual
Cortex

What happens if there is damage to the visual pathway? Different visual
problems will occur depending on where the damage is. The black bars
(labeled 1 through 5) indicate where damage may occur and the chart to the
right of the pathway indicates the resulting "blind" area (gray shading)
of the visual field.

Damage at site #1: this would be like losing sight in the left eye.
The entire left optic nerve would be cut and there would be a total loss
of vision from the left eye.

Damage at site #2: partial damage to the left optic nerve. Here,
information from the nasal visual field of the left eye (temporal part of
the left retina) is lost.

Damage at site #3: the optic chiasm would be damaged. In this case,
the temporal (lateral) portions of the visual field would be lost. The
crossing fibers are cut in this example.

Damage at site #4 and #5: damage to the optic tract (#4) or the fiber
tract from the lateral geniculate to the cortex (#5) can cause identical
visual loss. In this case, loss of vision of the right side.

Partial damage to these fiber tracts can cause other predictable
visual problems.